Totally agree that the transmission is the weak link in my truck. But, my 5.9 Cummins has a lot of latitude in it to get more power out of it with upgrades.

With the transmission, does anyone know if the Aisin transmission can be bolted to the 5.9 Cummins? If so, then I could swap out to an Aisin and then mod my engine to get the power I need.

The whole reason for asking about the differential was to determine if that would be the best place to cheaply get more torque for towing. If not, then I should concentrate instead on the engine and tranny.

And, BTW, the specs for my engine are about 300hp and 500 lb/ft of torque.

The swap should be the whole truck for a 2013 or newer 3500 SRW with Aisin. Spending big bucks on a 2003 is not the best plan.

Our 2001.5 with 4.10's towed well. The choice then was 3.54 or 4.10. Your 3.73 are a compromise between the two. If you had 3.54's then you would see a pretty good change, however with 3.73's, the change would not be worth the cost. It would cost north of 10k to put a Aisin in, and i am not sure anyone has worked out now to shift it.